1. Field of the Invention
The present invention relates to an image printing method and apparatus which can be suitably applied to a color digital copying machine, multifunction apparatus, and the like which print a color image based on color image information and output it.
2. Description of the Prior Art
Recently, a color image printing apparatus has been put into practice, which is designed to print a color image on the basis of red (R), green (G), and blue (B) image data acquired from a colored original image or receive image data from a printer controller (external apparatus) such as a server or personal computer (to be simply referred to as a PC hereinafter) to print a color image on the basis of the image data. A color image printing apparatus of this type is disclosed in Japanese Unexamined Patent Publication No. 2002-72771 (see pp. 14–25 and FIG. 5 of this reference).
FIG. 1 is a block diagram showing the schematic arrangement of a conventional color image printing apparatus 200. The color image printing apparatus 200 shown in FIG. 1 has a control unit 15. A color registration sensor 11, density (to be referred to as Dmax hereinafter) sensor 12, timer unit 13, counter unit 16, image printing unit 18′, and nonvolatile memory 36 are connected to the control unit 15. The image printing unit 18′ has an intermediate transfer belt 6 and convey driving unit 28. When a yellow (to be referred to as Y hereinafter) image printing unit is to be formed, the image printing unit 18′ includes a photosensitive drum 1Y, charging unit 2Y, exposure unit 3Y, developing unit 4Y, and the like for Y. Although not shown, this apparatus also includes magenta (to be simply referred to as M hereinafter), cyan (to be simply referred to as C hereinafter), and black (to be simply referred to as BK hereinafter) image printing units. The convey driving unit 28 drives the intermediate transfer belt 6 and photosensitive drum 1Y on the basis of a convey driving signal S4′. The convey driving signal S4′ is, for example, a signal which is corrected by a signal for adjusting the magnification of an image in the vertical direction. This signal is output from the control unit 15 to the convey driving unit 28.
In the image printing unit 18′, when the photosensitive drum 1Y is charged by the charging unit 2Y, the exposure unit 3Y irradiates the photosensitive drum 1Y with a laser beam having a predetermined intensity based on image printing data DOUT. As a result of this write processing, an electrostatic latent image is printed on the photosensitive drum 1Y. This electrostatic latent image is developed by the developing unit 4Y with Y toner. The Y image developed with this Y toner is transferred onto the intermediate transfer belt 6. Toner images of the respective colors are superposed into a color image on the intermediate transfer belt 6. The color image is transferred from the intermediate transfer belt 6 onto a paper sheet (not shown). This image is then fixed by the fixing unit (see Japanese Unexamined Patent Publication No. 2002-72771).
FIGS. 2A and 2B are views showing an example (first example) of vertical magnification adjustment for a toner image in the color image printing apparatus 200. In the correction image patterns shown in FIGS. 2A and 2B, the “+” symbols constitute a pattern for detecting the reduction of an image in the vertical direction (the sub-scanning direction in image printing). In this example, the “+” symbols are arranged in quincunxes.
A length L between the “+” symbols of a correction image pattern P1 shown in FIG. 2A which is expected by the control system should be equal to a length between the “+” symbols of a correction image pattern P2 shown in FIG. 2B which is actually printed on a paper sheet by the image printing system. However, image reduction occurs depending on the type or size of paper sheet. For example, in some case, a length L2 between the “+” symbols of the correction image pattern P2 actually printed on a paper sheet by the image printing system becomes larger than a length L1 between the “+” symbols of the correction image pattern P1 expected by the control system (L2>L1). For this reason, the vertical magnification of a toner image must be adjusted in accordance with the type or size of paper sheet.
Each of Japanese Unexamined Patent Publication Nos. 2002-258680 (see p. 3 and FIG. 2) and 06-289681 (see p. 2 and FIG. 2) discloses an image printing apparatus having a photosensitive drum changing function.
FIGS. 3A and 3B are views showing another example (second example) vertical magnification adjustment for a toner image in the color image printing apparatus 200 of this type. The elliptic hatched portions of an image print example P21 shown in FIG. 3A are a toner image printed on the intermediate transfer belt 6. In this example, three elliptic hatched portions are printed side by side in the direction (sub-scanning direction) in which the intermediate transfer belt 6 moves. In an image print example P22 shown in FIG. 3B, the circular hatched portions are obtained by transferring the toner image constituted by the elliptic hatched portions printed on the intermediate transfer belt 6 onto a paper sheet having a predetermined paper quality and size, and fixing the image. The image shown in FIG. 3B is a target image constituted by circular hatched portions.
When an image constituted by circular hatched portions is to be obtained on a paper sheet having a predetermined paper quality and size, the intermediate transfer belt 6 is controlled to increase its rotational speed on the basis of a vertical magnification adjustment value obtained in advance. With this operation, as shown in FIG. 3A, elliptic hatched portions are printed with respect to circular hatched portions expected by the control system. This makes it possible to adjust a reduction difference A between the image print example P21 and the image print example P22. In general, as the rotational speed of the intermediate transfer belt 6 is increased, an image is enlarged, and vice versa (see Japanese Unexamined Patent Publication Nos. 2002-258680 and 06-289681).
FIG. 4 is a flow chart showing an example of the operation of the conventional color image printing apparatus 200. According to the color image printing apparatus 200, a correction operation mode and print operation mode are prepared in advance. In the correction operation mode, a toner image is printed on the intermediate transfer belt 6 to correct the image printing system. In the print operation mode, the image printing system is operated to print an image on a paper sheet.
In the conventional color image printing apparatus 200, in step B1 in the flow chart shown in FIG. 4, control information is input (output) to the control unit 15. This control information is used to determine the necessity of the correction operation mode, and includes operating time data D7 and image printing count data D8 of the image printing unit 18′. The flow then advances to step B2, in which the control unit 15 outputs the convey driving signal S4′ to the convey driving unit 28 to rotate the intermediate transfer belt 6, the photosensitive drum 1Y, registration rollers 23, a fixing roller, and the like at rotational speeds set in consideration of a vertical magnification adjustment value.
In step B3, the control unit 15 checks whether the correction operation mode or the print operation mode is selected. In this case, the control unit 15 reads out a control target value DR from the nonvolatile memory 36, and compares the operating time data D7 obtained from the timer unit 13 with the control target value DR.
If the comparison result indicates that the operating time of the image printing unit 18′ exceeds the control target value DR, the correction operation mode is selected. The flow then advances to step B4 to execute correction operation. In the correction operation mode, the control unit 15 corrects a color registration positional shift, corrects the read timing of the Dmax sensor 12, or measures the density of a correction image on the intermediate transfer belt 6 using the Dmax sensor 12 after correction. On the basis of this measurement result, for example, the control unit 15 corrects the amount of charging by the charging unit 2Y for the Y image printing system or the laser power (Y laser) in the exposure unit 3Y. The flow then advances to step B7. Similar corrections are made for the remaining M, C, and BK image printing systems.
If the print operation mode is selected in step B3, the flow advances to step B5, in which the control unit 15 executes the print operation mode. In the image printing unit 18′, when the photosensitive drum 1Y is charged by the charging unit 2Y, the exposure unit 3Y irradiates the charged photosensitive drum 1Y with a laser beam having a predetermined intensity based on image printing data. As a result of this write processing, an electrostatic latent image is printed on the photosensitive drum 1Y. This electrostatic latent image is developed by the developing unit 4Y with Y toner. The Y image developed with this Y toner is transferred onto the intermediate transfer belt 6. Toner images of the respective colors are superposed into a color image on the intermediate transfer belt 6. The color image is transferred from the intermediate transfer belt 6 onto a paper sheet.
The flow then advances to step B6 to check whether or not there is a next print to be made, in the print operation mode. In the correction mode, it is checked whether or not there is another correction image to be printed. If there is a next print to be made or another correction operation to be performed, the flow returns to step B3 to repeat the above processing. If it is determined in step B6 that there is no next print to be made or another correction operation to be performed, the flow advances to step B7 to execute termination processing. In this termination processing, the apparatus enters the standby state after the lapse of a predetermined period of time. Alternatively, the image printing control is terminated upon detection of power-off information.
According to the conventional color image printing apparatus 200, after the control unit 15 rotates the intermediate transfer belt 6, photosensitive drum 1Y, registration rollers 23, fixing roller, and the like at rotational speeds set in consideration of a vertical magnification adjustment value in step B2 in FIG. 4, it is checked in step B3 whether the correction operation mode or print operation mode is selected by the control unit 15. Therefore, the following problems arise.
In the image correction operation mode as well, in which no image is actually printed on a paper sheet, since the intermediate transfer belt 6 and photosensitive drum 1Y are rotated in accordance with a vertical magnification adjustment value set in consideration of a predetermined paper quality and size, a toner image (correction image) which is made larger than the image to be actually printed on a paper sheet in consideration of image reduction is printed on the intermediate transfer belt 6 every time.
Consequently, a correction image such as a patch image becomes undesirably large in the correction operation mode, and the consumption of toner becomes always larger to the extent that the correction image becomes larger.
In addition, since a correction image is formed larger in the longitudinal direction of the intermediate transfer belt 6 than in the transverse direction, it takes a longer time for the correction image to pass under each sensor. This may hinder attempts to speed up the read timing correction processing of the Dmax sensor 12. This therefore may cause a deterioration in the correction precision of the read timing of the sensor system or hinder attempts to increase the precision in correcting the magnification of an image in the vertical direction.